The chemical mechanism of the enzyme yeast pyruvate decarboxylase will be probed by chemical and spectroscopic techniques. For this purpose the enzyme will be purified to homogeneity. The mechanism of E. coli pyruvate dehydrogenase multienzyme complex will be studied by a C-13 kinetic isotope effect measurement on the CO2 generated. Model studies on the role of the aminopyrimidine moiety in thiamin catalysed reactions will be continued.